Photodetectors and one or two dimensional arrays of the photodetectors comprised of a compound semiconductor material system have sensitivity in the near ultraviolet and the infrared regions that silicon detectors don't cover. They are in broad demand in fields such as sensing devices for optical communications and spectroscopic systems, or as infrared cameras for medical treatment, disaster prevention, industrial inspection, and others.
A photodiode, which has semiconductor P-N junction, is simple in its operating principle and excellent at quantitative performance, however, it produces very small current output for weak incident light, since only one pair of electron and hole is generated with one photon at most. Therefore, detection limit is determined by the noise characteristics of an external electric amplifier. Therefore, phototransistors having an internal amplification function have been developed also as compound semiconductor photodetectors. Even in a photodiode, demand on its excellent performance is still highly demanded, as it is easy to operate due to its 2 terminal device characteristics.
The performance of a photodiode is characterized by the detectable wavelength range, the quantum efficiency determined by the photo-induced carrier per photon, the dark current which determines the noise level, and the response speed, and the cross talk between adjacent elements when an array is constructed. Various improvements have been made on each of them so far.
A photodiode has two structure types. The one is a mesa structure in which a P-N junction is formed beforehand by crystal growth, and then, a photodiode element is formed by etching in the shape of an island. The other is a planar structure in which impurities are selectively doped in the depth direction from the surface of an n-type epitaxial layer to invert to the p-type, where the n layer is used as the cathode layer and the p layer is used as the anode layer. Although the mesa structure has an advantage in reducing a device size and floating capacitance, which is suitable for obtaining high-speed response, there is an issue of large surface leak current (dark current) generated at the P-N junction exposed on the sidewall of the island mesa structure. On the other hand, the planar structure has an advantage of suppressing the surface leak current, since the P-N junction including an optical absorption layer is not exposed to the surface. However, it also has large disadvantage, for example, additional processes are required for isolating adjacent elements adequately when an array is constructed, and it may become disadvantageous in manufacturing process in many cases.
In general, the crystal surface has large crystal defect density compared with the inside of the crystal. Therefore, the recombination and generation current becomes serious in case when the optical absorption layer with a narrow band gap is exposed to the surface and kept under the depletion condition. Then, dark current can be suppressed by the selective diffusion of impurities from the cap layer with a wide energy band gap, which forms the P-N junction on the surface in the wide energy band gap semiconductor.
According to this knowledge, as mentioned in the following document 1, there was an invention to suppress the dark current also in the photodiode which has a mesa structure. That is, after preparing the epitaxial structure where a photo-absorption layer with a narrow band gap is sandwiched by semiconductor layers with a wide energy band gap and forming a mesa structure, the whole mesa region and the mesa bottom of the semiconductor layer with a large band gap are selectively doped with impurities.    Document 1: U.S. Pat. No. 4,904,608.
On the other hand, in order to improve the quantum efficiency and the detectable wavelength range, it is effective to prepare a P-N junction at the shallow position in the depth direction from the surface. This is because light is substantially absorbed in the surface layer for the short wavelength light with energy higher than the band gap of the photo-absorption layer. And light-excited electron hole pairs cannot reach to the depletion layer which generates photo-induced current, if the P-N junction is located at a deep position. For example, in the case of a planar type PD, though it is not a mesa structure, the following document 2 discloses an attempt of setting the junction depth at about 0.3 μm deep by shortening the Zn diffusion time and extending the detection wavelength to the short wavelength side up to 0.7 μm. On the other hand, in the following document 3, although the P-N junction is formed at a shallow position by Be ion implantation, dark current is comparatively large caused by crystal defects created by high energy ion implantation.    Document 2: Shuzo Kagawa, etc. “Wide-Wavelength InGaAs/InP PIN photodiodes Sensitive from 0.7 to 1.6 μm”, Japanese Journal of Applied Physics 28 (1989) pp. 1843-1846.    Document 3: U.S. Pat. No. 4,887,138.
In contrast to such photodiodes, the heterojunction bipolar phototransistor (HPT) is highly advantageous for obtaining higher sensitivity and has lower noise characteristics than the avalanche photodiode (APD) which also has the amplification function of photoelectric current. Therefore, HPTs have been continuously studied until now since the 1980s. However, in the HPT, the effect of the recombination current generated between the emitter and the base junction becomes prominent in the low illumination range which results in low emitter current, and reduces the current gain at the low collector current conditions. Moreover, the dark current generated in the reversely biased base collector junction drifts to the base layer and is amplified same as the photo-induced current so that the dark current becomes the source of noise and reduces the sensitivity of photodetection.
Therefore, suppressing the recombination current between the emitter and the base, and the generation current between the emitter and the collector are important for realizing a highly sensitive HPT. Regarding the suppression of the recombination current between the emitter and the base, the improvement technique taken for the heterojunction bipolar transistor (HBT) aiming at amplification of electric signals can be applied effectively. For example, in the following documents 4 and 5, such technologies are disclosed that the surface recombination of carriers is suppressed by regrowth of a highly resistive or thin p-type semiconductor at the sidewall of the emitter and base region of the HBT formed in ridge geometry. Especially, when the regrowth layer is made with a wide energy band gap semiconductor as a so-called embedded structure, a barrier can be formed, which prevents minority carriers to move toward the surface. However, only with these techniques disclosed by documents 4 and 5, crystal defects are created in the regrown interface due to oxidization or disorder of stoichiometry at the sidewall of the mesa structure formed by etching. And they become major source of surface recombination current or ohmic current. This regrowth method is generally better than the dielectric passivation; however, it has not brought the best result yet.    Document 4: JP-A 1987-141769.    Document 5: JP-A 1987-141770.
In addition, as disclosed in the following documents 6 for example, there is the technology of diffusing Zn from the emitter surface to reach to the base layer aiming to lower the external base resistance. The contact resistance between the base electrode and the intrinsic base layer inside of the device tends to be high since the base layer is thin. This is called “Craft Base Structure” nowadays and it became a common knowledge. According to such a structure as stated also in the following document 7, carrier recombination at the exposed sidewall of the base is suppressed, and an improvement of current gain is recognized as the base emitter junction exposed on the ridge sidewall has moved to the inside of the device.    Document 6: JP-A 1986-280665.    Document 7: JP-A 1987-139354.
Furthermore, not for the HBT but especially for the HPT, in order to enhance the transistor amplification function for very weak incident light, it is reported in the following document 8 that the recombination of photo-generated charges at the surface of the base region is suppressed and an improvement in the transistor characteristic is recognized by protecting the surface (which intersects the sidewall perpendicularly and parallel to the main surface of substrate) with the wide band gap emitter rather than exposing the narrow gap base, as a photosensitive layer upward to the surface.    Document 8: Shin-Wei Tan, etc. “Characterization and Modeling of Three-Terminal Heterojunction Phototransistors Using and InGaP Layer for Passivation”, IEEE Transactions on Electron Devices, Vol. 52, No. 2, pp. 204-210, February 2005.
As stated in the following document 9, the structure called “Punch Through HPT” is proposed which sets the base region depleted by lowering the base carrier concentration and it produces about 10 times higher sensitivity compared to a conventional HPT mode. This is owing to the effect that influence of the recombination current between the emitter and the base is reduced by maintaining the emitter idling current without base bias current, and that the base potential can be lowered with small photo-generated charges, which is brought by the fact and the emitter-base and the base-collector junction capacitances are decreased.
Document 9: Y. Wang, etc. “High gain and wide dynamic range puch-through heterojunction phototransistors”, J. Appl. Phys. 74 (11) pp. 6987-6981, December 1993.
On the other hand, the argument from another viewpoint is made in the following document 10. That is, in the physical measurement by a discrete element or one-dimensional array, it takes about 1 second for a human to recognize the data, and as for an image sensor, a frame rate is at most about 1/30 seconds in the case of video imaging. Therefore, it may be more desirable to be able to integrate light signal from several milliseconds to several seconds. Concerning the applications for physics and chemistry measurements or enhancement of infrared camera sensitivity, it is said that it is not always necessary to take an operation speed into consideration.
In HBTs, it is important to reduce the base resistance in order to improve the frequency characteristic, and the base carrier concentration is usually set to the order of 1019 cm−3. However, the minority carrier recombination rate increases in the base region with the high base doping concentration, and the low frequency gain of a transistor is rather deteriorated.
Thus, in the usual HBT, the high frequency characteristic becomes the most important issue, and the base layer is formed thin, and the carrier concentration is set high in order to reduce the base resistance. While in the HPT photodetector array for obtaining a higher sensitivity, the base concentration of the order of 1017 cm−3 can be said to be advantageous from the view point of crystal quality.    Document 10: U.S. Pat. No. 7,067,853
In order to obtain an efficient photo-response to the wavelength shorter than the band gap energy, generally, it is necessary to form a P-N junction at shallowest possible depth. This is to make the incident light penetrate and generate the electron and hole pairs within the minority carrier diffusion length from the P-N junction. Moreover, since the surface recombination effect is large on the device surface, it is effective to cover the device surface with the wide band gap semiconductor which has a low recombination rate.
By using a wide band gap semiconductor as a window, the electron and hole pairs, which are generated inside the photo-absorption layer of the narrowband gap semiconductor, are prevented from moving to the surface, and recombination of the electron and hole pairs can be suppressed.
For example, in the case of the heterojunction having an InP as a wide band gap semiconductor and InGaAs as a narrow band gap semiconductor, the ratio of a short wavelength light reaching to the InGaAs through the InP is estimated. The required design condition is α (λ) L<1, where L is the thickness of InP, and α (λ) is the absorption coefficient for the detection wavelength λ. For example, the optical absorption coefficient of the InP at the wavelength at λ=0.6 μm is 6.42×104 cm−1 so that the thickness to make a L=1 is 156 nm. The optical absorption coefficient of InP at λ=0.5 μm is 1.09×105 cm−1 so that the thickness to make αL=1 is 92 nm, similarly 18 nm at λ=0.4 μm.
However, the idea of increasing the detectable wavelength range was not well considered for the structures such as abovementioned conventional planar type PIN photodiodes or the mesa type PIN photodiodes disclosed in the above documents. Usually selective diffusion of Zn was carried out all over the light receiving area through a comparatively thick n-InP layer. This is because the surface (the light receiving surface) where Zn diffusion was carried out has a high Zn concentration and a lot of surface defects, and the minority carrier life becomes short. Therefore, the P-N junction where photo carriers are generated was necessary to form enough space from the surface in order to obtain an efficient photodiode. Another reason for the deep Zn diffusion depth is to make the reverse breakdown voltage higher by increasing the radius of curvature and reducing the electric field intensity of the diffused front edge. It is also favorable to ensure the electric conductivity of the surface.
Furthermore, it is necessary to perform a diffusion process at relatively high temperature (500-600 degrees C.) in order to ensure the repeatability of Zn diffusion conditions and suppress the generation of crystal defects. To ensure the repeatability of the diffusion conditions, it is necessary to perform diffusion for at least 10 minutes by taking the heat up time of a sample into consideration. In the above condition, the diffusion depth resulted in about 0.5 in the case of InP, and about 0.2 μm in InGaAs. It is difficult, therefore, to form the P-N junction using a thinner InP than this by the Zn diffusion through the whole light receiving surface to the depth direction. In the case of thin InP, the effect of Zn accumulation in the InP-InGaAs hetero interface is recognized, which in turn makes the precise control of doping concentration profile difficult as stated in the following document 11.    Document 11: F. Dildey, etc. “Segregation of Zinc in InGaAs/InP Heterostructures During Diffusion: Experiment and Numerical Modeling”, Japanese Journal of Applied Physics vol. 29, No. 5 (1990) pp. 810-812.
In other words, in the conventional planar type PDs or the PDs disclosed by the abovementioned document 1, the specified wavelength range could not be extended enough to the short wavelength side, since to form the P-N junction in a very shallow depth position was not well considered or difficult. In the abovementioned document 2, although the location of the P-N junction is set to about 0.3 μm in the depth direction, it is not shallow enough for the required wavelength range. As stated in the abovementioned document 3, when a P-N junction is formed at a shallow depth position by the ion implantation of Be, the leakage current increases about one hundred times compared to a Zn-diffused planar type PIN diode due to the crystal defects created by the ion-implantation as already stated.
In general, in the case of the PIN photodiode with an InP/InGaAs system, it is desirable to form a p-type InGaAs and an n-type InGaAs layer, both with low impurity concentration of comparatively long minority carrier lifetime under a thin p-type InP cap layer in order to obtain high quantum efficiency and wide wavelength spectra range, where the thickness of the p-type InGaAs layer should be less than the sum of the minority carrier diffusion length of the p-type InGaAs layer and a depletion layer width of about 1 μm. This structure is easily realized with the metal organic chemical vapor deposition (MOCVD) or the molecular beam epitaxy (MBE). In this case, mesa etching is necessary to remove the surface p-type layer to isolate the devices since the P-N junction is formed all over the wafer parallel to the principal surface of the substrate. In the case of the HPT instead of the PD, a device isolation is needed by mesa etching after growing a base layer of a p-type narrow band gap semiconductor sandwiched by an emitter layer of a n-type wide band gap semiconductor and a collector layer of a n-type wide band gap semiconductor epitaxially. However, even if the device isolation is done comparatively easily, it induces increase of the leakage current since the sidewall of the P-N junction is exposed after mesa etching. This issue must be solved.
Generally sources of the dark current are the current generation resulting from crystal defects, the thermally excited current corresponding to the band gap energy, and electric field induced current components such as avalanche effect. As for the site of the dark current generated in the photodetector, there are two sources, the bulk component generated inside of the device and the surface component generated on the surface of the device. The main factors of the bulk component are the generation current resulting from crystal defects and the thermally excited current corresponding to the band gap energy. Therefore, the reduction of the bulk dark current is rather difficult as it is uniquely determined by the physical properties and crystalline quality of the materials to be used. In addition to the improvement of the crystalline quality in general, the suppression of dark current has been performed by cooling a photodetector, so far.
On the other hand, the surface dark current component has the generation and recombination current at the surface, and the leakage current of ohmic nature.
As the HPT structures mentioned above or the conventional photo FET structures having a mesa structure where a semiconductor material exposes to the sidewall, the crystal surface conditions are greatly affected by the manufacturing skills such as etching and passivation technology. In other words, the surface dark current resulting from the generation and recombination current, and the ohmic leakage current are largely dependent on the manufacturing process. This causes the deterioration and non-uniformity of the device characteristics, which results in lowering the manufacturing yield.
Moreover, when a device structure has an acute-angle shape, the dark current is affected by the punch-through phenomenon due to amplification of electric field or by electric field induced avalanche effect. Since the crystalline conditions of the surface is easily affected in case of a narrow band-gap material, the longer the detectable wavelength of the photodetector, the more serious the effect of the crystal surface on the dark current.
According to the well known SRH (Shockley-Read-Hall) statistics, electron and hole pairs are generated when the product of the electron and hole concentration are smaller than that of the thermal equilibrium condition, and in the contrary, electronic hole pairs recombine when the product of the electron and hole concentration are larger than that of the thermal equilibrium condition. Moreover, in the case of depletion condition, the carrier generation rate becomes higher in the narrow band-gap semiconductor with larger intrinsic carrier concentration ni. In case of the P-N junction of a mesa structure, most part of the electron hole pairs are generated at the surface defects in the exposed device side surface particularly in the narrow band-gap materials under the depletion condition. The holes generated in this exposed side surface are led to the anode, and becomes the dark current component. In case of the HPT, dark current occurs in the exposed side surface of the depleted collector region, and holes disappear in the side surface of the base region which reduces the current amplification factor β.
Moreover, when providing the abovementioned punch-through type HPT or improving the current amplification factor beta by blocking holes and leading electrons to the collector layer smoothly, it requires precisely controlled band-gap profiles or the base layer with comparatively low carrier concentration by fine epitaxial techniques. However, the Zn diffusion and ion implantation techniques which are generally performed as the traditional impurity doping method can possibly disturb not only carrier concentration but also material composition profiles as stated in the following document 12 is employed.    Document 12: K. Goto, “Zn-Diffusion-Induced Disordering of InGaAs/AlGaInAs Multiple Quantum Well and Its Application to Long-wavelength Laser”, Japanese Journal of Applied Phys. Vol. 33 (1994) pp. 5774-5778.
Incorporating various knowledge mentioned above, this invention has its object to solve or ease the defects resided in the above-mentioned conventional devices. This invention provides photodetectors having structurally advantageous mesa structure particularly of HPT composed with compound semiconductor materials, which are built with an effective surface current blocking structure (SCB) and designed not to serve the surface region as a current path in order to suppress the dark current caused by generation and recombination current at the surface. Therefore, the photodetectors provided with this invention have higher sensitivity and wider wavelength range than conventional devices.